The control of friction and wear of metal mechanical components that are in sliding or rolling-sliding contact is of great importance in the design and operation of many machines and mechanical systems. For example, many steel-rail and steel-wheel transportation systems including freight, passenger and mass transit systems suffer from the emission of high noise levels and extensive wear of mechanical components such as wheels, rails and other rail components such as ties. The origin of such noise emission, and the wear of mechanical components may be directly attributed to the frictional forces and behaviour that are generated between the wheel and the rail during operation of the system. Furthermore, it has been observed that the control of friction also results in a reduction of lateral forces, and reduction in energy consumption.
In a dynamic system wherein a wheel rolls on a rail, there is a constantly moving zone of contact. For purposes of discussion and analysis, it is convenient to treat the zone of contact as stationary while the rail and wheel move through the zone of contact. When the wheel moves through the zone of contact in exactly the same direction as the rail, the wheel is in an optimum state of rolling contact over the rail. In such a case, no appreciable friction exists between the wheel and the rail. However, because the wheel and the rail are profiled, often misaligned and subject to motions other than strict rolling, the respective velocities at which the wheel and the rail move through the zone of contact are not always the same. This is often observed when fixed-axle railcars negotiate curves wherein true rolling contact can only be maintained on both rails if the inner and the outer wheels rotate at different peripheral speeds. This is not possible on most fixed-axle railcars. Thus, under such conditions, the wheels undergo a combined rolling and sliding movement relative to the rails. Sliding movement may also arise when traction is lost on inclines thereby causing the driving wheels to slip.
The magnitude of the sliding movement is roughly dependent on the difference, expressed as a percentage, between the rail and wheel velocities at the point of contact. This percentage difference is termed creepage.
At creepage levels larger than about 1%, appreciable frictional forces are generated due to sliding, and these frictional forces result in noise and wear of components (H. Harrison, T. McCanney and J. Cotter (2000), Recent Developments in COF Measurements at the Rail/Wheel Interface, Proceedings The 5th International Conference on Contact Mechanics and Wear of Rail/Wheel Systems CM 2000 (SEIKEN Symposium No. 27), pp. 30-34, which is incorporated herein by reference). The noise emission is a result of a negative friction characteristic that is present between the wheel and the rail system. A negative friction characteristic is one wherein friction between the wheel and rail generally decreases as the creepage of the system increases in the region where the creep curve is saturated. Theoretically, noise and wear levels on wheel-rail systems may be reduced or eliminated by making the mechanical system very rigid, by reducing the frictional forces between moving components to very low levels or by changing the friction characteristic from a negative to a positive one, that is by increasing friction between the rail and wheel in the region where the creep curve is saturated. Unfortunately, it is often impossible to impart greater rigidity to a mechanical system, such as in the case of a wheel and rail systems used by most trains. Alternatively, reducing the frictional forces between the wheel and the rail may greatly hamper adhesion and braking and is not always suitable for rail applications. In many situations, imparting a positive frictional characteristic between the wheel and rail is effective in reducing noise levels, wear of components, decreasing drawbar force reduction, lateral force reduction, or a combination thereof.
U.S. Pat. No. 5,308,516, U.S. Pat. No. 5,173,204 and WO 90/15123 relate to solid friction control compositions having high and positive friction characteristics. These compositions display increased friction as a function of creepage, and comprise resins to impart the solid consistency of these formulations. The resins employed included amine and polyamide epoxy resins, polyurethane, polyester, polyethylene or polypropylene resins.
EP 0372559 relates to solid coating compositions for lubrication which are capable of providing an optimum friction coefficient to places where it is applied, and at the same time are capable of lowering abrasion loss.
U.S. Pat. No. 6,649,573 and US 2004/0043908 relate to solid lubricants comprising a thermoplastic polymeric carrier (low or high density polyethylene), an inorganic and organic powder lubricant, optionally a synthetic extreme pressure anti-wear liquid oil and optionally an optical brightener. Similarly, U.S. Pat. No. 4,915,856, U.S. Pat. No. 5,435,925 provide solid lubricants comprising a thermoplastic polymeric carrier and optionally an oil, and a solid lubricant. These compositions are provided in a stick for application between the wheel and rail in wheel-rail systems. However, the solid lubricant stick exhibits extreme variability of consumption rate with varying temperature, a consistently low stick melting point and variability in lubricant loading. As a result, the resin may extrude between the flange of the wheel and the rail gauge face onto the top of the rail resulting in undesired top-or-rail contamination. Furthermore, as the thermoplastic polymeric carrier is a low or high density polyethylene, the solid lubricant sticks are prepared using an extrusion process and solid sticks so produced comprise flat ends. During use, as the stick is consumed, the end of the solid stick is not retained within the applicator which results in waste of the stick. The wasted portion of the stick will eventually fall out from the gap between the end of the applicator and the wheel, and may obstruct devices along a rail system, for example switches, and also has environmental implications.